Mycobacterium tuberculosis (Mtb) causes more adult deaths worldwide than any other bacterium, and many strains are resistant to all first-line drugs. Although HIV infection is associated with a high rate of active tuberculosis (TB), Mtb can cause disease in people with normal immune systems, implying that Mtb can evade the immune response. We tested the hypothesis that Mtb inhibits effector mechanisms of the immune response, and found that Mtb inhibits macrophage (MO) transcriptional responses to IFNgamma. Studies during the present funding period revealed that Mtb utilizes 3 distinct mechanisms to inhibit MO responses to IFNg: an Mtb lipoprotein (LP) activates a TLR2-dependent mechanism for inhibition; Mtb peptidoglycan (PG) activates a TLR2- and MyD88-independent mechanism, and Mtb induces secretion of interleukin-6, which also inhibits MO responses to IFNg. We also found that Mtb-LP and Mtb-PG block IFNg priming of MO to kill Mtb, implying that inhibition of MO responses to IFNg contributes to persistence and progression of TB. The overall goal of the experiments proposed in this application is to define the mechanisms of inhibition of MO responses to IFNg, in order to develop the means to overcome the inhibition and enhance the efficacy of the immune response to Mtb. Specific Aim 1 is to define the structural features of Mtb PG that account for its 40-fold greater potency compared to E. coli PG for inhibition of MO responses to IFNg; Specific Aim 2 is to determine the contributions of Mtb lipoproteins, peptidoglycan, and IL-6 to modulating gene expression and Mtb killing in MO, and Specific Aim 3 is to define the early, intermediate, and late steps in Mtb PG-induced inhibition of MO responses to IFNg, and compare them to the steps required for Mtb LP- and interleukin 6-initiated inhibition. The proposed experiments will provide high-resolution understanding of the mechanisms used by Mtb to block MO responses to IFNg, and will provide a basis for interventions to overcome the block and enhance immunity to Mtb.